What are enzymes? The role of enzymes in the human body. Enzymes and their role in the human body Structure and mechanism of action of enzymes

Without enzymes, a person will not be viable, since the body needs protein molecules for all important metabolic processes and healthy digestion.

Enzymes in the human body have a protein structure. You can imagine them as catalysts of the human body, which ensure the functioning of all metabolic processes. They stimulate numerous biochemical reactions and ensure that the body gets the nutrients it needs from food.

Mechanism of action

Enzymes break down nutrients so that they can be used by the body. As a result, nutrients from food are introduced into the body.

In fact, enzymes are very smart! Each of the supposed 10,000 different types of enzymes in the body has a specific function: it acts on a specific substrate. Thus, protein-digesting enzymes exclusively digest proteins and do not dissolve fat.

To change its function, an enzyme can briefly combine with another substrate, resulting in an enzyme-substrate complex. Subsequently, it returns to the original structure.

The main groups of enzymes in the body

Enzymes fall into three categories: digestive, nutritional, and metabolic enzymes. While digestive and metabolic enzymes are produced by the body itself, the body receives food enzymes from human consumption of raw food.

1. Digestive. These proteins are produced in the pancreas, stomach, small intestine, and salivary glands in the mouth. There they separate the food molecules into their basic building blocks and thus ensure their availability for the metabolic process.

A particularly important organ for the production of many digestive enzymes is the pancreas. It produces amylase, which converts carbohydrates into simple sugars, lipase, which creates glycerol and simple fatty acids from fats, and protease, which forms amino acids from proteins.

2. Food. This group of enzymes is found in raw fresh foods. Food enzymes act as digestive enzymes. Benefit: They directly aid in the digestion of food.

With the consumption of fresh fruits and raw vegetables, food enzymes in the body digest up to 70% of food. Heat destroys them, so it is important to eat food raw. It should be as diverse as possible to ensure the supply of different enzymes.

Bananas, pineapples, figs, pears, papaya and kiwi are especially rich in them. Among the vegetables stand out broccoli, tomatoes, cucumbers and zucchini.

3. Metabolic. This group of enzymes is produced in cells, organs, bones and blood. Only because of their presence can the heart, kidneys and lungs work. Metabolic enzymes ensure that nutrients are effectively supplied from food.

Thus, they deliver vitamins, minerals, phytonutrients and hormones to the body.

Effect on the skin

Hard-working enzyme biocatalysts in the body help not only inside the body, but also outside. People who suffer from acne or have sensitive skin can improve their appearance with their help. To speed up the process, special enzyme peels are used. They are usually made up of fruit enzymes.

Such procedures remove dead skin cells and remove excess sebum. Enzymatic peels are freely sold and act very gently on the skin. However, they should not be used more than once a week.

Enzymes (Enzymes) are specific proteins, biologically active organic substances that speed up chemical reactions in the cell. The huge role of enzymes in the body. They can increase the reaction rate by more than ten times. It is simply necessary for the normal functioning of the cell. And enzymes are involved in every reaction.

In the body of all living beings, including even the most primitive microorganisms, enzymes have been found. Enzymes, due to their catalytic activity, are very important for the normal functioning of our body systems.

Key enzymes in the body

The life of the human body is based on thousands of chemical reactions occurring in cells. Each of them is carried out with the participation of special accelerators - biocatalysts, or enzymes.

Enzymes act as catalysts in almost all biochemical reactions occurring in living organisms. By 2013, over 5,000 different enzymes had been described.

Modern science knows about two thousand biocatalysts. Let's focus on the so-called key enzymes . These include the most essential biocatalysts for the life of the organism, the “breakage” of which, as a rule, leads to the occurrence of diseases. We strive to answer the question: how does this enzyme act in a healthy body and what happens to it in the process of human disease?

It is known that the most important biopolymers that form the basis of all living things (all the constituent parts of the cells of our body and all enzymes are built from them) are of a protein nature. In turn, proteins consist of simple nitrogenous compounds - amino acids, linked by chemical bonds - peptide bonds. In the body, there are special enzymes that split these bonds by attaching water molecules (hydrolysis reaction). Such enzymes are called peptide hydrolases. Under their influence, chemical bonds between amino acids are broken in protein molecules and fragments of protein molecules are formed - peptides, consisting of a different number of amino acids. Peptides, having high biological activity, can even cause poisoning of the body. Eventually, when exposed to peptide hydrolases, peptides either lose or significantly reduce their biological activity.

In 1979, Professor VN Orekhovich and his students managed to discover, isolate in pure form and study in detail the physical, chemical and catalytic properties of one of the peptide hydrolases, previously unknown to biochemists. Now it is included in the international list under the name of the enzyme carboxycatepsin. Research has made it possible to get closer to answering the question: why does a healthy body need carboxycatepsin and what can happen as a result of certain changes in its structure.

It turned out that carboxycatepsin is involved both in the formation of the angiotensin B peptide, which increases blood pressure, and in the destruction of another peptide, bradykinin, which, on the contrary, has the property of lowering blood pressure.

Thus, carboxycatepsin turned out to be a key catalyst involved in the work of one of the most important biochemical systems of the body - the blood pressure regulation system. The greater the activity of carboxycatepsin, the higher the concentration of angiotensin II and the lower the concentration of bradykinin, and this, in turn, leads to an increase in blood pressure. It is not surprising that in people suffering from hypertension, the activity of carboxy-catepsin in the blood is increased. The definition of this indicator helps doctors evaluate the effectiveness of therapeutic measures, predict the course of the disease.

Is it possible to inhibit the action of carboxythepsin directly in the human body and thereby achieve a decrease in blood pressure? Studies conducted at our institute have shown that in nature there are peptides that are able to bind to carboxycatepsin without being hydrolyzed, and thereby deprive it of the ability to perform its inherent function.

Currently, work is underway on the synthesis of artificial blockers (inhibitors) of carboxycatepsin, which are supposed to be used as new therapeutic agents to combat hypertension.

Other important key enzymes involved in the biochemical transformations of nitrogenous substances in the human body include amine oxidases. Without them, the oxidation reactions of the so-called biogenic amines, to which many chemical transmitters of nerve impulses belong - neurotransmitters, cannot do. Breakdowns of amine oxidases lead to disorders of the functions of the central and peripheral nervous system; chemical blockers of amine oxidases are already used in clinical practice as therapeutic agents, for example, in depressive states.

In the process of studying the biological functions of amine oxidases, it was possible to discover their previously unknown property. It turned out that certain chemical changes in the molecules of these enzymes are accompanied by qualitative changes in their catalytic properties. Thus, monoamine oxidases that oxidize biogenic monoamines (for example, the well-known neurotransmitters noradrenaline, serotonin, and dopamine) partially lose their inherent properties after treatment with oxidizing agents. But on the other hand, they discover a qualitatively new ability to destroy diamines, some amino acids and amino sugars, nucleotides and other nitrogenous compounds necessary for the life of the cell. Moreover, it is possible to transform monoamine oxidases not only in a test tube (that is, in cases where researchers experiment with purified enzyme preparations), but also in an animal body, in which various pathological processes are preliminarily modeled.

In the cells of the human body, monoamine oxidases are included in the composition of biological membranes - semi-permeable partitions that serve as cell membranes and divide each of them into separate compartments where certain reactions take place. Biomembranes are especially rich in easily oxidized fats, which are in a semi-liquid state. Many diseases are accompanied by the accumulation of excessive amounts of fat oxidation products in biomembranes. Excessively oxidized (peroxidized), they disrupt both the normal permeability of membranes and the normal functioning of the enzymes that make up their composition. These enzymes include monoamine oxidases.

In particular, during radiation injury, fats are overoxidized in the biomembranes of cells of the bone marrow, intestines, liver and other organs, and monoamine oxidases not only partially lose their useful activity, but also acquire a qualitatively new property that is harmful to the body. They begin to destroy nitrogenous substances vital for the cell. The property of mono-amine oxidases to transform their biological activity is manifested both in experiments with purified enzyme preparations and in a living organism. Moreover, it turned out that the therapeutic agents used in the fight against radiation injuries also prevent the development of qualitative changes in enzymes.

This very important property - the reversibility of the transformation of monoamine oxidases - was established in experiments, during which researchers learned not only to prevent the transformation of enzymes, but also to eliminate violations, returning the functions of catalysts to normal and achieving a certain therapeutic effect.

While we are talking about animal experiments. However, today there is every reason to believe that the activity of amine oxidases also changes in the human body, in particular, in atherosclerosis. Therefore, the study of the properties of amine oxidases, as well as chemicals that can be used to influence their activity in the human body for therapeutic purposes, is currently continuing with particular perseverance.

And the last example. It is well known what an important role carbohydrates play in the life of our body, and, consequently, the key enzymes that accelerate their biochemical transformations. These catalysts include the enzyme gamma-amylase discovered at our institute; he takes part in the splitting of chemical bonds between glucose molecules (complex glycogen molecules are built from them). Congenital absence or insufficiency of gamma-amylase leads to disruption of the normal biochemical transformations of glycogen. Its content in the cells of the vital organs of the child increases, they lose the ability to perform their inherent functions. All these changes characterize the most severe disease - glycogenosis.

Other enzymes are also involved in the biochemical transformations of glycogen.

Their congenital deficiency also leads to glycogenoses. In order to timely and accurately recognize what type of glycogenosis a child suffers from (and this is important for choosing a method of treatment and predicting the course of the disease), it is necessary to study the activity of a number of enzymes, including gamma-amylase. Developed in the 1970s at the Institute of Biological and Medical Chemistry of the USSR Academy of Medical Sciences, the methods of differential laboratory and chemical diagnosis of glycogenosis are still used in clinical practice.

According to Professor V.Z. GORKINA

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Millions of chemical reactions take place in the cell of any living organism. Each of them is of great importance, so it is important to maintain the speed of biological processes at a high level. Almost every reaction is catalyzed by its own enzyme. What are enzymes? What is their role in the cell?

Enzymes. Definition

The term "enzyme" comes from the Latin fermentum - leaven. They may also be called enzymes, from the Greek en zyme, "in yeast."

Enzymes are biologically active substances, so any reaction that occurs in a cell cannot do without their participation. These substances act as catalysts. Accordingly, any enzyme has two main properties:

1) The enzyme speeds up the biochemical reaction, but is not consumed.

2) The value of the equilibrium constant does not change, but only accelerates the achievement of this value.

Enzymes speed up biochemical reactions by a thousand, and in some cases a million times. This means that in the absence of an enzymatic apparatus, all intracellular processes will practically stop, and the cell itself will die. Therefore, the role of enzymes as biologically active substances is great.

A variety of enzymes allows you to diversify the regulation of cell metabolism. In any cascade of reactions, many enzymes of various classes take part. Biological catalysts are highly selective due to the specific conformation of the molecule. Since enzymes in most cases are of a protein nature, they are in a tertiary or quaternary structure. This is again explained by the specificity of the molecule.

Functions of enzymes in the cell

The main task of the enzyme is to speed up the corresponding reaction. Any cascade of processes, from the decomposition of hydrogen peroxide to glycolysis, requires the presence of a biological catalyst.

The correct functioning of enzymes is achieved by high specificity for a particular substrate. This means that a catalyst can only speed up a certain reaction and no other, even a very similar one. According to the degree of specificity, the following groups of enzymes are distinguished:

1) Enzymes with absolute specificity, when only one single reaction is catalyzed. For example, collagenase breaks down collagen and maltase breaks down maltose.

2) Enzymes with relative specificity. This includes substances that can catalyze a certain class of reactions, such as hydrolytic cleavage.

The work of a biocatalyst begins from the moment of attachment of its active site to the substrate. In this case, one speaks of a complementary interaction like a lock and a key. Here we mean the complete coincidence of the shape of the active center with the substrate, which makes it possible to accelerate the reaction.

The next step is the reaction itself. Its speed increases due to the action of the enzymatic complex. In the end, we get an enzyme that is associated with the products of the reaction.

The final stage is the detachment of the reaction products from the enzyme, after which the active center again becomes free for the next work.

Schematically, the work of the enzyme at each stage can be written as follows:

1) S + E ——> SE

2) SE ——> SP

3) SP ——> S + P, where S is the substrate, E is the enzyme, and P is the product.

Enzyme classification

In the human body, you can find a huge number of enzymes. All knowledge about their functions and work was systematized, and as a result, a single classification appeared, thanks to which it is easy to determine what this or that catalyst is intended for. Here are the 6 main classes of enzymes, as well as examples of some of the subgroups.

Oxidoreductases.

Enzymes of this class catalyze redox reactions. There are 17 subgroups in total. Oxidoreductases usually have a non-protein part, represented by a vitamin or heme.

Among oxidoreductases, the following subgroups are often found:

a) Dehydrogenases. The biochemistry of dehydrogenase enzymes consists in the elimination of hydrogen atoms and their transfer to another substrate. This subgroup is most often found in the reactions of respiration, photosynthesis. The composition of dehydrogenases necessarily contains a coenzyme in the form of NAD / NADP or flavoproteins FAD / FMN. Often there are metal ions. Examples are enzymes such as cytochrome reductase, pyruvate dehydrogenase, isocitrate dehydrogenase, and many liver enzymes (lactate dehydrogenase, glutamate dehydrogenase, etc.).

b) Oxidases. A number of enzymes catalyze the addition of oxygen to hydrogen, as a result of which the reaction products can be water or hydrogen peroxide (H 2 0, H 2 0 2). Examples of enzymes: cytochrome oxidase, tyrosinase.

c) Peroxidases and catalases are enzymes that catalyze the breakdown of H 2 O 2 into oxygen and water.

d) oxygenases. These biocatalysts accelerate the addition of oxygen to the substrate. Dopamine hydroxylase is one example of such enzymes.

2. Transferases.

The task of the enzymes of this group is to transfer radicals from the donor substance to the recipient substance.

a) methyltransferase. DNA methyltransferases, the main enzymes that control the process of nucleotide replication, play an important role in the regulation of the nucleic acid.

b) Acyltransferases. Enzymes of this subgroup transport the acyl group from one molecule to another. Examples of acyltransferases: lecithincholesterol acyltransferase (transfers a functional group from a fatty acid to cholesterol), lysophosphatidylcholine acyltransferase (an acyl group is transferred to lysophosphatidylcholine).

c) Aminotransferases - enzymes that are involved in the conversion of amino acids. Examples of enzymes: alanine aminotransferase, which catalyzes the synthesis of alanine from pyruvate and glutamate by amino group transfer.

d) Phosphotransferases. Enzymes of this subgroup catalyze the addition of a phosphate group. Another name for phosphotransferases, kinases, is much more common. Examples are enzymes such as hexokinases and aspartate kinases, which add phosphorus residues to hexoses (most often glucose) and to aspartic acid, respectively.

3. Hydrolases - a class of enzymes that catalyze the cleavage of bonds in a molecule, followed by the addition of water. Substances that belong to this group are the main digestive enzymes.

a) Esterases - break ester bonds. An example is lipases, which break down fats.

b) Glycosidases. The biochemistry of enzymes of this series consists in the destruction of glycosidic bonds of polymers (polysaccharides and oligosaccharides). Examples: amylase, sucrase, maltase.

c) Peptidases are enzymes that catalyze the breakdown of proteins into amino acids. Peptidases include enzymes such as pepsins, trypsin, chymotrypsin, carboxypeptidase.

d) Amidases - cleave amide bonds. Examples: arginase, urease, glutaminase, etc. Many amidase enzymes are found in

4. Lyases - enzymes that are similar in function to hydrolases, however, when cleaving bonds in molecules, water is not consumed. Enzymes of this class always contain a non-protein part, for example, in the form of vitamins B1 or B6.

a) Decarboxylases. These enzymes act on the C-C bond. Examples are glutamate decarboxylase or pyruvate decarboxylase.

b) Hydratases and dehydratases - enzymes that catalyze the reaction of splitting C-O bonds.

c) Amidine-lyases - destroy C-N bonds. Example: arginine succinate lyase.

d) P-O lyase. Such enzymes, as a rule, cleave off the phosphate group from the substrate substance. Example: adenylate cyclase.

The biochemistry of enzymes is based on their structure

The abilities of each enzyme are determined by its individual, unique structure. Any enzyme is, first of all, a protein, and its structure and degree of folding play a decisive role in determining its function.

Each biocatalyst is characterized by the presence of an active center, which, in turn, is divided into several independent functional areas:

1) The catalytic center is a special region of the protein, along which the enzyme is attached to the substrate. Depending on the conformation of the protein molecule, the catalytic center can take a variety of forms, which must fit the substrate in the same way as a lock to a key. Such a complex structure explains what is in the tertiary or quaternary state.

2) Adsorption center - acts as a "holder". Here, first of all, there is a connection between the enzyme molecule and the substrate molecule. However, the bonds formed by the adsorption center are very weak, which means that the catalytic reaction at this stage is reversible.

3) Allosteric centers can be located both in the active center and over the entire surface of the enzyme as a whole. Their function is to regulate the functioning of the enzyme. Regulation occurs with the help of inhibitor molecules and activator molecules.

Activator proteins, binding to the enzyme molecule, accelerate its work. Inhibitors, on the contrary, inhibit catalytic activity, and this can occur in two ways: either the molecule binds to the allosteric site in the region of the active site of the enzyme (competitive inhibition), or it attaches to another region of the protein (noncompetitive inhibition). considered more efficient. After all, this closes the place for the binding of the substrate to the enzyme, and this process is possible only in the case of almost complete coincidence of the shape of the inhibitor molecule and the active center.

An enzyme often consists not only of amino acids, but also of other organic and inorganic substances. Accordingly, the apoenzyme is isolated - the protein part, the coenzyme - the organic part, and the cofactor - the inorganic part. The coenzyme can be represented by carbohydrates, fats, nucleic acids, vitamins. In turn, the cofactor is most often auxiliary metal ions. The activity of enzymes is determined by its structure: additional substances that make up the composition change the catalytic properties. Various types of enzymes are the result of a combination of all the listed factors of complex formation.

Enzyme regulation

Enzymes as biologically active substances are not always necessary for the body. The biochemistry of enzymes is such that they can harm a living cell in case of excessive catalysis. To prevent the harmful effects of enzymes on the body, it is necessary to somehow regulate their work.

Since enzymes are of a protein nature, they are easily destroyed at high temperatures. The process of denaturation is reversible, but it can significantly affect the work of substances.

pH also plays a big role in regulation. The greatest activity of enzymes, as a rule, is observed at neutral pH values (7.0-7.2). There are also enzymes that work only in an acidic environment or only in an alkaline one. So, in cellular lysosomes, a low pH is maintained, at which the activity of hydrolytic enzymes is maximum. If they accidentally enter the cytoplasm, where the environment is already closer to neutral, their activity will decrease. Such protection against "self-eating" is based on the features of the work of hydrolases.

It is worth mentioning the importance of coenzyme and cofactor in the composition of enzymes. The presence of vitamins or metal ions significantly affects the functioning of some specific enzymes.

Enzyme nomenclature

All enzymes of the body are usually named depending on their belonging to any of the classes, as well as on the substrate with which they react. Sometimes, not one, but two substrates are used in the name.

Examples of the names of some enzymes:

Liver enzymes: lactate dehydrogenase, glutamate dehydrogenase.
Full systematic name of the enzyme: lactate-NAD+-oxidoreduct-ase.

There are also trivial names that do not adhere to the rules of nomenclature. Examples are digestive enzymes: trypsin, chymotrypsin, pepsin.

Enzyme Synthesis Process

The functions of enzymes are determined at the genetic level. Since a molecule is by and large a protein, its synthesis exactly repeats the processes of transcription and translation.

The synthesis of enzymes occurs according to the following scheme. First, information about the desired enzyme is read from DNA, as a result of which mRNA is formed. Messenger RNA codes for all the amino acids that make up the enzyme. Enzyme regulation can also occur at the DNA level: if the product of the catalyzed reaction is sufficient, gene transcription stops and vice versa, if there is a need for a product, the transcription process is activated.

After the mRNA has entered the cytoplasm of the cell, the next stage begins - translation. On the ribosomes of the endoplasmic reticulum, a primary chain is synthesized, consisting of amino acids connected by peptide bonds. However, the protein molecule in the primary structure cannot yet perform its enzymatic functions.

The activity of enzymes depends on the structure of the protein. On the same ER, protein twisting occurs, as a result of which first secondary and then tertiary structures are formed. The synthesis of some enzymes stops already at this stage, however, to activate the catalytic activity, it is often necessary to add a coenzyme and a cofactor.

In certain areas of the endoplasmic reticulum, the organic components of the enzyme are attached: monosaccharides, nucleic acids, fats, vitamins. Some enzymes cannot work without the presence of a coenzyme.

The cofactor plays a decisive role in the formation Some of the functions of enzymes are available only when the protein reaches the domain organization. Therefore, the presence of a quaternary structure is very important for them, in which the connecting link between several protein globules is a metal ion.

Multiple forms of enzymes

There are situations when it is necessary to have several enzymes that catalyze the same reaction, but differ from each other in some parameters. For example, an enzyme can work at 20 degrees, but at 0 degrees it will no longer be able to perform its functions. What should a living organism do in such a situation at low ambient temperatures?

This problem is easily solved by the presence of several enzymes at once, catalyzing the same reaction, but operating under different conditions. There are two types of multiple forms of enzymes:

Isoenzymes. Such proteins are encoded by different genes, consist of different amino acids, but catalyze the same reaction.
True plural forms. These proteins are transcribed from the same gene, but peptides are modified on the ribosomes. As a result, several forms of the same enzyme are obtained.

As a result, the first type of multiple forms is formed at the genetic level, while the second type is formed at the post-translational level.

Importance of enzymes

In medicine, it comes down to the release of new drugs, in which the substances are already in the right quantities. Scientists have not yet found a way to stimulate the synthesis of missing enzymes in the body, but today drugs are widely used that can temporarily make up for their deficiency.

Various enzymes in the cell catalyze a wide variety of life-sustaining reactions. One of these enisms are representatives of the group of nucleases: endonucleases and exonucleases. Their job is to maintain a constant level of nucleic acids in the cell, removing damaged DNA and RNA.

Do not forget about such a phenomenon as blood clotting. Being an effective measure of protection, this process is under the control of a number of enzymes. The main one is thrombin, which converts the inactive protein fibrinogen into active fibrin. Its threads create a kind of network that clogs the site of damage to the vessel, thereby preventing excessive blood loss.

Enzymes are used in winemaking, brewing, obtaining many fermented milk products. Yeast can be used to produce alcohol from glucose, but an extract from them is sufficient for the successful flow of this process.

Interesting facts you didn't know

All enzymes of the body have a huge mass - from 5,000 to 1,000,000 Da. This is due to the presence of protein in the molecule. For comparison: the molecular weight of glucose is 180 Da, and carbon dioxide is only 44 Da.

To date, more than 2,000 enzymes have been discovered that have been found in the cells of various organisms. However, most of these substances are not yet fully understood.

Enzyme activity is used to produce effective laundry detergents. Here, enzymes perform the same role as in the body: they break down organic matter, and this property helps in the fight against stains. It is recommended to use a similar washing powder at a temperature not exceeding 50 degrees, otherwise the denaturation process may occur.

According to statistics, 20% of people around the world suffer from a lack of any of the enzymes.

The properties of enzymes have been known for a very long time, but only in 1897 people realized that not the yeast itself, but an extract from their cells, could be used to ferment sugar into alcohol.

Biological chemistry Lelevich Vladimir Valeryanovich

The mechanism of action of enzymes

In any enzymatic reaction, the following stages are distinguished:

E+S? ?E+P

where E is the enzyme, S is the substrate, is the enzyme-substrate complex, P is the product.

The mechanism of action of enzymes can be considered from two positions: from the point of view of changes in the energy of chemical reactions and from the point of view of events in the active center.

Energy changes in chemical reactions

Any chemical reactions proceed, obeying two basic laws of thermodynamics: the law of conservation of energy and the law of entropy. According to these laws, the total energy of a chemical system and its environment remains constant, while the chemical system tends to reduce order (increase entropy). To understand the energy of a chemical reaction, it is not enough to know the energy balance of the substances entering and leaving the reaction. It is necessary to take into account the energy changes in the process of a given chemical reaction and the role of enzymes in the dynamics of this process.

The more molecules have an energy exceeding the level of Ea (activation energy), the higher the rate of a chemical reaction. The rate of a chemical reaction can be increased by heating. This increases the energy of the reacting molecules. However, high temperatures are detrimental to living organisms, so enzymes are used in the cell to speed up chemical reactions. Enzymes provide a high rate of reactions under optimal conditions existing in the cell by lowering the level of Ea. Thus, enzymes lower the height of the energy barrier, as a result of which the number of reactive molecules increases, and, consequently, the reaction rate increases.

The role of the active site in enzymatic catalysis

As a result of research, it was shown that the enzyme molecule, as a rule, is many times larger than the substrate molecule undergoing chemical transformation by this enzyme. Only a small part of the enzyme molecule comes into contact with the substrate, usually from 5 to 10 amino acid residues, which form the active site of the enzyme. The role of the remaining amino acid residues is to ensure the correct conformation of the enzyme molecule for the optimal course of the chemical reaction.

The active site at all stages of enzymatic catalysis cannot be considered as a passive site for substrate binding. It is a complex molecular "machine" using a variety of chemical mechanisms that promote the transformation of a substrate into a product.

In the active center of the enzyme, the substrates are located in such a way that the functional groups of the substrates participating in the reaction are in close proximity to each other. This property of the active center is called the effect of approach and orientation of the reactants. Such an ordered arrangement of substrates causes a decrease in entropy and, as a consequence, a decrease in the activation energy (Ea), which determines the catalytic efficiency of enzymes.

The active center of the enzyme also contributes to the destabilization of interatomic bonds in the substrate molecule, which facilitates the course of a chemical reaction and the formation of products. This property of the active center is called the substrate deformation effect.

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ChapterIV.3.

Enzymes

Metabolism in the body can be defined as the totality of all chemical transformations undergone by compounds coming from outside. These transformations include all known types of chemical reactions: intermolecular transfer of functional groups, hydrolytic and non-hydrolytic splitting of chemical bonds, intramolecular rearrangement, new formation of chemical bonds and redox reactions. Such reactions proceed in the body at an extremely high rate only in the presence of catalysts. All biological catalysts are substances of a protein nature and are called enzymes (hereinafter F) or enzymes (E).

Enzymes are not components of reactions, but only accelerate the achievement of equilibrium by increasing the rate of both direct and reverse transformations. The acceleration of the reaction occurs due to a decrease in the activation energy - the energy barrier that separates one state of the system (the initial chemical compound) from another (the reaction product).

Enzymes speed up a wide variety of reactions in the body. So, quite simple from the point of view of traditional chemistry, the reaction of splitting off water from carbonic acid with the formation of CO 2 requires the participation of an enzyme, because without it, it proceeds too slowly to regulate the pH of the blood. Thanks to the catalytic action of enzymes in the body, it becomes possible to carry out such reactions that would go hundreds and thousands of times slower without a catalyst.

Enzyme Properties

1. Influence on the rate of a chemical reaction: enzymes increase the rate of a chemical reaction, but they themselves are not consumed.

The reaction rate is the change in the concentration of the reaction components per unit time. If it goes in the forward direction, then it is proportional to the concentration of the reactants; if it goes in the opposite direction, then it is proportional to the concentration of the reaction products. The ratio of the rates of forward and reverse reactions is called the equilibrium constant. Enzymes cannot change the values of the equilibrium constant, but the state of equilibrium in the presence of enzymes comes faster.

2. The specificity of the action of enzymes. In the cells of the body, 2-3 thousand reactions take place, each of which is catalyzed by a certain enzyme. The specificity of the action of an enzyme is the ability to accelerate the course of one particular reaction without affecting the rate of others, even very similar ones.

Distinguish:

Absolute– when F catalyzes only one specific reaction ( arginase- breakdown of arginine)

Relative(group special) - F catalyzes a certain class of reactions (eg hydrolytic cleavage) or reactions involving a certain class of substances.

The specificity of enzymes is due to their unique amino acid sequence, which determines the conformation of the active center that interacts with the reaction components.

A substance whose chemical transformation is catalyzed by an enzyme is called substrate ( S ) .

3. The activity of enzymes is the ability to accelerate the reaction rate to varying degrees. Activity is expressed in:

1) International units of activity - (IU) the amount of the enzyme catalyzing the conversion of 1 μM of the substrate in 1 min.

2) Katalakh (cat) - the amount of catalyst (enzyme) capable of converting 1 mol of substrate in 1 s.

3) Specific activity - the number of units of activity (any of the above) in the test sample to the total mass of protein in this sample.

4) Less often, molar activity is used - the number of substrate molecules converted by one enzyme molecule per minute.

activity depends on temperature . This or that enzyme shows the greatest activity at an optimum temperature. For F of a living organism, this value is within +37.0 - +39.0° C, depending on the type of animal. With a decrease in temperature, Brownian motion slows down, the diffusion rate decreases and, consequently, the process of complex formation between the enzyme and the reaction components (substrates) slows down. In case of temperature increase above +40 - +50° With the enzyme molecule, which is a protein, undergoes a process of denaturation. At the same time, the rate of the chemical reaction drops noticeably (Fig. 4.3.1.).

Enzyme activity also depends on medium pH . For most of them, there is a certain optimal pH value at which their activity is maximum. Since the cell contains hundreds of enzymes and each of them has its own opt pH limits, the change in pH is one of the important factors in the regulation of enzymatic activity. So, as a result of one chemical reaction with the participation of a certain enzyme, the pH opt of which lies in the range of 7.0 - 7.2, a product is formed, which is an acid. In this case, the pH value shifts to the region of 5.5 - 6.0. The activity of the enzyme sharply decreases, the rate of product formation slows down, but another enzyme is activated, for which these pH values are optimal, and the product of the first reaction undergoes further chemical transformation. (Another example about pepsin and trypsin).

The chemical nature of enzymes. The structure of the enzyme. Active and allosteric centers

All enzymes are proteins with molecular weights ranging from 15,000 to several million Da. According to the chemical structure, they are simple enzymes (consist only of AA) and complex enzymes (have a non-protein part or a prosthetic group). The protein portion is called apoenzyme, and non-protein, if it is covalently linked to an apoenzyme, then it is called coenzyme, and if the bond is non-covalent (ionic, hydrogen) - cofactor . The functions of the prosthetic group are as follows: participation in the act of catalysis, contact between the enzyme and the substrate, stabilization of the enzyme molecule in space.

Inorganic substances usually act as a cofactor - ions of zinc, copper, potassium, magnesium, calcium, iron, molybdenum.

Coenzymes can be considered as an integral part of the enzyme molecule. These are organic substances, among which there are: nucleotides ( ATP, UMF, etc.), vitamins or their derivatives ( TDF- from thiamine ( IN 1), FMN- from riboflavin ( IN 2), coenzyme A- from pantothenic acid ( IN 3), NAD, etc.) and tetrapyrrole coenzymes - hemes.

In the process of catalysis of the reaction, not the entire enzyme molecule comes into contact with the substrate, but a certain part of it, which is called active center. This zone of the molecule does not consist of a sequence of amino acids, but is formed when the protein molecule is twisted into a tertiary structure. Separate sections of amino acids approach each other, forming a certain configuration of the active center. An important structural feature of the active center is that its surface is complementary to the surface of the substrate; AA residues of this zone of the enzyme are able to enter into chemical interaction with certain groups of the substrate. It can be imagined that the active site of the enzyme matches the structure of the substrate like a key and a lock.

IN active center two zones are distinguished: binding center, responsible for the attachment of the substrate, and catalytic center responsible for the chemical transformation of the substrate. The composition of the catalytic center of most enzymes includes such AAs as Ser, Cys, His, Tyr, Lys. Complex enzymes in the catalytic center have a cofactor or coenzyme.

In addition to the active center, a number of enzymes are equipped with a regulatory (allosteric) center. Substances that affect its catalytic activity interact with this zone of the enzyme.

The mechanism of action of enzymes

The act of catalysis consists of three successive stages.

1. Formation of an enzyme-substrate complex during interaction through the active center.

2. The binding of the substrate occurs at several points of the active center, which leads to a change in the structure of the substrate, its deformation due to a change in the bond energy in the molecule. This is the second stage and is called substrate activation. When this occurs, a certain chemical modification of the substrate and its transformation into a new product or products.

3. As a result of such a transformation, the new substance (product) loses its ability to be retained in the active center of the enzyme and the enzyme-substrate, or rather, the enzyme-product complex, dissociates (disintegrates).

Types of catalytic reactions:

A + E \u003d AE \u003d BE \u003d E + B

A + B + E \u003d AE + B \u003d ABE \u003d AB + E

AB + E \u003d ABE \u003d A + B + E, where E is an enzyme, A and B are substrates, or reaction products.

Enzymatic effectors - substances that change the rate of enzymatic catalysis and thereby regulate metabolism. Among them are distinguished inhibitors - slowing down the rate of reaction and activators - accelerating the enzymatic reaction.

Depending on the mechanism of inhibition of the reaction, competitive and non-competitive inhibitors are distinguished. The structure of the competitive inhibitor molecule is similar to the structure of the substrate and coincides with the surface of the active center like a key with a lock (or almost coincides). The degree of this similarity may even be higher than with the substrate.

If A + E \u003d AE \u003d BE \u003d E + B, then I + E \u003d IE¹

The concentration of the enzyme capable of catalysis decreases and the rate of formation of reaction products drops sharply (Fig. 4.3.2.).

A large number of chemicals of endogenous and exogenous origin (i.e., formed in the body and coming from outside - xenobiotics, respectively) act as competitive inhibitors. Endogenous substances are regulators of metabolism and are called antimetabolites. Many of them are used in the treatment of oncological and microbial diseases, maybe. they inhibit key metabolic reactions of microorganisms (sulfonamides) and tumor cells. But with an excess of the substrate and a low concentration of a competitive inhibitor, its action is canceled.

The second type of inhibitors is non-competitive. They interact with the enzyme outside the active site, and an excess of substrate does not affect their inhibitory ability, as is the case with competitive inhibitors. These inhibitors interact either with certain groups of the enzyme (heavy metals bind to the thiol groups of Cys) or most often with the regulatory center, which reduces the binding ability of the active center. The actual process of inhibition is the complete or partial suppression of enzyme activity while maintaining its primary and spatial structure.

There are also reversible and irreversible inhibition. Irreversible inhibitors inactivate the enzyme by forming a chemical bond with its AA or other structural components. Usually this is a covalent bond with one of the sites of the active center. Such a complex practically does not dissociate under physiological conditions. In another case, the inhibitor disrupts the conformational structure of the enzyme molecule - causing its denaturation.

The action of reversible inhibitors can be removed by an excess of the substrate or by the action of substances that change the chemical structure of the inhibitor. Competitive and non-competitive inhibitors are in most cases reversible.

In addition to inhibitors, activators of enzymatic catalysis are also known. They are:

1) protect the enzyme molecule from inactivating effects,

2) form a complex with the substrate, which more actively binds to the active center of F,

3) interacting with an enzyme having a quaternary structure, they separate its subunits and thereby open access for the substrate to the active center.

Distribution of enzymes in the body

Enzymes involved in the synthesis of proteins, nucleic acids and energy metabolism enzymes are present in all cells of the body. But cells that perform special functions also contain special enzymes. So the cells of the islets of Langerhans in the pancreas contain enzymes that catalyze the synthesis of the hormones insulin and glucagon. Enzymes that are peculiar only to the cells of certain organs are called organ-specific: arginase and urokinase- liver, acid phosphatase- prostate. By changing the concentration of such enzymes in the blood, the presence of pathologies in these organs is judged.

In the cell, individual enzymes are distributed throughout the cytoplasm, others are embedded in the membranes of mitochondria and the endoplasmic reticulum, such enzymes form compartments, in which certain, closely related stages of metabolism occur.

Many enzymes are formed in cells and secreted into the anatomical cavities in an inactive state - these are proenzymes. Often in the form of proenzymes, proteolytic enzymes (break down proteins) are formed. Then, under the influence of pH or other enzymes and substrates, their chemical modification occurs and the active center becomes available to the substrates.

There are also isoenzymes - enzymes that differ in molecular structure, but perform the same function.

Nomenclature and classification of enzymes

The name of the enzyme is formed from the following parts:

1. the name of the substrate with which it interacts

2. the nature of the catalyzed reaction

3. the name of the enzyme class (but this is optional)

4. suffix -aza-

pyruvate - decarboxyl - aza, succinate - dehydrogen - aza

Since about 3 thousand enzymes are already known, they must be classified. Currently, an international classification of enzymes has been adopted, which is based on the type of catalyzed reaction. There are 6 classes, which in turn are divided into a number of subclasses (in this book they are presented only selectively):

1. Oxidoreductases. Catalyze redox reactions. They are divided into 17 subclasses. All enzymes contain a non-protein part in the form of heme or derivatives of vitamins B 2, B 5. The substrate undergoing oxidation acts as a hydrogen donor.

1.1. Dehydrogenases remove hydrogen from one substrate and transfer it to other substrates. Coenzymes NAD, NADP, FAD, FMN. They accept on themselves the hydrogen cleaved off by the enzyme, turning into the reduced form (NADH, NADPH, FADH) and transfer it to another enzyme-substrate complex, where it is given away.

1.2. Oxidase - catalyzes the transfer of hydrogen to oxygen with the formation of water or H 2 O 2. F. Cytochromoxysdase respiratory chain.

RH + NAD H + O 2 = ROH + NAD + H 2 O

1.3. Monooxidases - cytochrome P450. According to its structure, both hemo- and flavoprotein. It hydroxylates lipophilic xenobiotics (by the mechanism described above).

1.4. PeroxidasesAnd catalase- catalyze the decomposition of hydrogen peroxide, which is formed during metabolic reactions.

1.5. Oxygenases - catalyze the reactions of oxygen addition to the substrate.

2. Transferases - catalyze the transfer of various radicals from the donor molecule to the acceptor molecule.

BUT but+ E + B = E but+ A + B = E + B but+ A

2.1. Methyltransferase (CH 3 -).

2.2 Carboxyl- and carbamoyltransferases.

2.2. Acyltransferases - Coenzyme A (acyl group transfer - R-C=O).

Example: synthesis of the neurotransmitter acetylcholine (see chapter "Protein metabolism").

2.3. Hexosyl transferases catalyze the transfer of glycosyl residues.

Example: the splitting of a glucose molecule from glycogen under the action of phosphorylase.

2.4. Aminotransferases - transfer of amino groups

R 1- CO - R 2 + R 1 - CH - NH 3 - R 2 \u003d R 1 - CH - NH 3 - R 2 + R 1 - CO - R 2

They play an important role in the transformation of AK. The common coenzyme is pyridoxal phosphate.

Example: alanine aminotransferase(AlAT): pyruvate + glutamate = alanine + alpha-ketoglutarate (see chapter "Protein metabolism").

2.5. Phosphotransferesis (kinase) - catalyze the transfer of a phosphoric acid residue. In most cases, ATP is the phosphate donor. Enzymes of this class are mainly involved in the process of glucose breakdown.

Example: Hexo (gluco) kinase.

3. Hydrolases - catalyze hydrolysis reactions, i.e. splitting of substances with addition at the place of breaking the bond of water. This class includes mainly digestive enzymes, they are one-component (do not contain a non-protein part)

R1-R2 + H 2 O \u003d R1H + R2OH

3.1. Esterases - break down essential bonds. This is a large subclass of enzymes that catalyze the hydrolysis of thiol esters, phosphoesters.
Example: NH 2 ).

Example: arginase(urea cycle).

4. Liases - catalyze the reactions of cleavage of molecules without the addition of water. These enzymes have a non-protein part in the form of thiamine pyrophosphate (B 1) and pyridoxal phosphate (B 6).

4.1. C-C bond lyases. They are commonly referred to as decarboxylases.

Example: pyruvate decarboxylase.

5.Isomerases - catalyze isomerization reactions.

Example: phosphopentose isomerase, pentose phosphate isomerase(enzymes of the non-oxidative branch of the pentose phosphate pathway).

6. Ligases catalyze the synthesis of more complex substances from simple ones. Such reactions proceed with the expenditure of ATP energy. Synthetase is added to the name of such enzymes.

LITERATURE TO THE CHAPTER IV.3.

1. Byshevsky A. Sh., Tersenov O. A. Biochemistry for a doctor // Ekaterinburg: Ural worker, 1994, 384 p.;

2. Knorre D. G., Myzina S. D. Biological chemistry. - M .: Higher. school 1998, 479 pp.;

3. Filippovich Yu. B., Egorova T. A., Sevastyanova G. A. Workshop on General Biochemistry // M.: Prosveschenie, 1982, 311 pp.;

4. Lehninger A. Biochemistry. Molecular bases of the structure and functions of the cell // M.: Mir, 1974, 956 p.;

5. Pustovalova L.M. Workshop on biochemistry // Rostov-on-Don: Phoenix, 1999, 540 p.